1. Field of the Invention
The present invention relates to a control device for controlling a sensorless induction motor by estimating a slip frequency of the motor and estimating a motor speed using the slip frequency instead of detecting the motor speed using a speed sensor.
2. Description of the Related Art
A variety of methods for controlling an induction motor which has no speed sensor are proposed in Japanese Patent No. 3067659 (JP3067659B) for example. In these methods, a slip of the motor is estimated from a value of a current flowing through windings in the motor, an estimated speed of the induction motor is calculated, and feedback control of the induction motor is carried out. A voltage command signal value produced by the feedback control becomes a desired command signal value if speed feedback data used in the feedback control is accurate.
FIG. 1 is a diagram showing a configuration of a sensorless induction motor control device which does not has a function of correcting the slip frequency according to the related art. The sensorless induction motor control device shown in FIG. 1 carries out vector control which independently controls a torque command signal value (IQ command signal value) IQcmd and an excitation command signal value (ID command signal value) IDcmd. When three-phase current feedback from an induction motor 10 is carried out, a three-phase-to-two-phase conversion unit 12 generates current feedback signals IDfb and IQfb by DQ conversion. Then, a voltage command signal value calculation unit 14 generates voltage command signal values VDcmd and VQcmd using a D-phase current feedback signal IDfb, Q-phase current feedback signal IQfb, current command signal value, and excitation frequency command signal value ω1cmd. A current compensator 16 generates an excitation frequency command signal value ω1cmd based on a difference between the IQ command signal value IQcmd and the current feedback signal IQfb. An excitation phaseθ is calculated by integrating the excitation frequency command signal value ω1cmd. The two-phase-to-three-phase conversion unit 18 generates a three-phase voltage command signal value by inverse DQ conversion using the voltage command signal value VDcmd, voltage command signal value VQcmd, and excitation phaseθ. The induction motor 10 is driven by the three-phase voltage command signal. A slip frequency estimation unit 20 estimates a slip frequency estimation value ωs^, which corresponds to a slip of the induction motor 10, in accordance with an equation ωs^=K×IQfb/IDfb, where K represents a constant. A subtractor 22 calculates a difference between the slip frequency estimation value ωs^ and excitation frequency command signal value ω1cmd. This difference corresponds to the speed estimation value ωr^ of the speed of the induction motor 10. A speed compensator 24 carries out speed control based on a speed command signal value ωrcmd and the speed estimation value ωr^.
In the sensorless induction motor control device shown in FIG. 1, the slip frequency estimation value ωs^ may differ from an actual value of the slip frequency when the speed of the induction motor 10 is high. As a result, the speed estimation value ωr^ will differ from an actual value of the speed of the induction motor 10, the excitation frequency command signal value ω1cmd will differ from an actual value of the excitation frequency, the voltage command signal value will become lower than an ideal value of voltage applied to the induction motor 10, and thus a desired output of the induction motor 10 cannot be obtained.
In other words, when the speed of the induction motor 10 is high, there is an error in the speed estimation value, a current cannot be generated at a desired phase, and thus an output of the induction motor 10 is reduced.
Therefore, an object of the present invention is to provide a sensorless induction motor control device which has a function of correcting a slip frequency, and is capable of improving the output of the motor by detecting error in the speed estimation value and correcting the slip frequency estimation value in an appropriate way.